The present disclosure relates to techniques for enhancing circuit efficiency and reducing cost of a flyback-type DC voltage conversion circuit.
A voltage conversion circuit for converting a DC input voltage into a desired DC voltage (a “DC—DC converter”) is used for a lighting circuit for a discharge lamp, such as a metal halide lamp. A flyback-type configuration, which may be used as the voltage conversion circuit, includes a transformer and a switching element provided on the primary side thereof and is arranged to control an output by changing a duty ratio of a driving signal for the element. That is, the flyback-type voltage conversion circuit is configured such that the switching element is coupled to the primary winding of the conversion transformer to perform on/off control of the element. A rectifying diode and a smoothing capacitor are provided on the secondary side of the transformer.
As disclosed in Japanese patent document JP-A-8-195290, a switching element may be operated so that current flowing into the transformer is placed in a boundary state.
For situations in which the switching element is placed in an on-state when energy accumulated in the transformer is completely discharged on the secondary side of the transformer (i.e., a current boundary mode), the switching element is placed in the on-state after the current flowing into the rectifying diode becomes zero ampere. Thus, advantageously, power loss does not occur at a reverse recovery time, and electric efficiency is good (i.e., power loss is small) when switching with a relatively high frequency (e.g., several hundred kilo-hertz or more).
A problem that may occur is that the switching frequency for realizing the current boundary mode is not always constant, but changes due to the influence of the variation of an input voltage and the variation of a load.
For example, in the case of a lighting circuit for a discharge lamp, an electric power applied to the discharge lamp may differ between a transient state just after lighting the discharge lamp serving as a load and a steady lighting state of the discharge lamp. The frequency at which the element is placed in the current boundary mode varies due to the influence of the variation of the battery voltage or the applied electric power.
In order to control the switching frequency so that the target state is close to the current boundary state, a discharge termination time point where the secondary current of the transformer becomes zero must be detected so that the switching frequency can be controlled in response to the detection. However, a detection circuit for detecting the discharge termination time point is complex, and may typically requires a circuit element having a high voltage-tolerance. Difficulties arise in connection with miniaturization and cost reduction of the circuit.
Accordingly, it is desirable to improve the control in the current boundary state in a flyback-type DC voltage conversion circuit, without significantly increasing the cost.